Typically such a bearing supports the rotating load of a moving element with respect to a ground element. One of the races, constitutes or is fixed to the moving element and the other is fixed to the ground element, typically by an interference press fit. The weight of the load is taken vertically on the upward facing side of the fixed race or bush, via the rolling elements where provided. Typically, a race which is fixed to the load rotates in use, so that the load is transmitted progressively and repeatedly around the entire circumference thereof. However the race or bush which is fixed to the ground element, being relatively stationary, has weight taken continuously by the same upwards facing portion; conversely the downwards facing portion takes no weight. As a result wear of the relatively fixed race is asymmetric, and the bearing may require replacement more frequently than if wear had been distributed around the entire circumference of the fixed race. Non-vertical loads may be transmitted to the same sector of a relatively stationary bearing, for example due to the separating forces on a pair of parallel geared shafts.
The problem of asymmetric wear is most apparent in bearings which support sustained unidirectional loads and/or support high bending forces in the vertical plane, such as hub bearings of wind turbines. The traditional solution is to employ larger bearings of high precision, but also to accept that more frequent replacement may be necessary. Large bearings are difficult and expensive to manufacture, and tend also to suffer disproportionate deterioration due to vibration compared with small bearings.
What is required is an arrangement capable of eliminating this kind of asymmetric wear profile, but which is adaptable to standard types of bearing, particularly rolling element bearings, and particularly to the fixed and moving elements which transmit loads via the bearing.
A further problem is sustained static pressurisation of bearing lubricant between the metal surfaces of the bearing. This occurs in the region of the upper side of the fixed race of the bearing arrangement when it is static. Over time, hydrogen atoms may escape from hydrocarbon molecules in the lubricant and react with the metal surfaces resulting in hydrogen embrittlement of the metal surfaces, which increases the incidence of failure.
What is required is an arrangement capable of limiting the exposure of the metal surfaces to prolonged contact with static pressurized lubricant.
One solution to these problems is to provide a bearing assembly comprising an inner race, and an outer race concentric with the inner race, one of said races being adapted to idle in rotation on a ground element.
This arrangement is applicable to a plain bearing, but is particularly suitable for a rolling element bearing having rolling elements between the inner and outer races.
The bearing is thus arranged so that the race of the ground element is adapted to idle on its support. The idling speed is preferable imperceptible, and may be as low as a few revolutions over the rotation and life of the bearing. Idling may be in the range 0.1 degree per day to 0.1 revolutions per minute.
Faster idling may be permissible according to the type and size of bearing, and kind of installation. In any event idling above 50 rpm, alternatively one tenth of hub speed, is not envisaged. Furthermore this solution is particularly suitable for large bearings, having a shaft diameter in excess of 250 mm.
In order to idle, the race of the ground element slides on or in the ground element, rather than being an interference fit, and the corresponding diameter of this race may be dimensioned accordingly. Thus where the inner race is mounted on the ground element, the inner diameter is slightly increased; where the outer race is on the ground element, the outer diameter is slightly reduced. Alternatively the ground element may be sized as a relatively loose fit in or on the adjacent race.
The race of the ground element should not rotate at a speed sufficient to cause wear of the ground element or of the race itself, and is preferably at least an order of magnitude less than the speed at which such wear is measurable over the life of the bearing.
The idling rotation may be continuous. However intermittent motion is also possible, and may be suitable where the bearing can tolerate a fixed position of one race for a pre-determined period—for example up to 5 years. Reversing and reciprocating motion is also envisaged.
It would be desirable to regulate and/or control idling in order to allow intelligent relative positioning of the races.